Reversible hammermills are particularly well adapted for processing coal of varying moisture content and hardness into a uniformly sized, fine product of the type required for cyclone furnace installations. Thus, for many years, most if not all of the coal fed to cyclone furnaces in the U.S. has been processed through such mills. The equipment is also used in coal plants and other systems requiring fine product sizes.
Gradual development of the state of the art with respect to this equipment is reflected in U.S. Pat. Nos. 2,149,571, 2,170,407, 2,471,068, 2,478,733, 2,514,111, 2,767,929, 2,819,027, 2,977,055, 3,035,782, 3,083,921, 3,465,973, 3,593,931, 3,617,007 and others.
The material reduction elements of these mills usually include a rotor mounted in the unit for rotation about an axis which is usually horizontal. The rotor comprises a shaft and hammers, including pivoting hammers, ring hammers, radial paddles or other impact members which protrude outwardly, i.e. in a direction which includes a radially outward component. Such hammers or other impact members are usually mounted in one or more circular or staggered arrays about the shaft. For instance, fixed paddles may be mounted in circular or staggered (e.g., helical) arrays on a common shaft. Pivoting hammers and ring hammers may be similarly mounted on sub-shafts secured to a main shaft by disks or spiders. These arrays rotate with the shaft or main shaft as the case may be, and the impact members have peripheral surfaces or edges which define a hammer circle upon rotation of the shaft.
Such units are provided with means for introducing feed particulates, such as coal, rock, other minerals or other materials of varying size and composition. For example, a typical reversible hammermill operating in a cyclone furnace system may receive sub-bituminous coal in pieces having dimensions in the range of about 3"-6".times.0". The feed particulates are usually introduced to the rotor from outside the hammer circle. This may, for example, be accomplished by a chute which, in a reversible hammermill, is typically centered above the rotor. Thus introduced, the material approaches the hammer circle with a component of motion directed radially inward with respect to the axis. The portion or arc of the hammer circle within which feed particulates normally first encounter the rotor is referred to herein as the in-feed position.
As is usual in such equipment, the first encounter between a feed particulate and a hammer often results in some breaking of the particulate into sub-particles, some of which may be above and below the upper particle size limit desired in the final product. The hammer flings such sub-particles and any initially uncrushed particulates outward, typically with an approximately tangential motion, against an impact breaker member. This may be a plate or casting, usually free of product screening openings, which may be supported by a housing within which the rotor is mounted.
The impact breaker member is typically mounted opposite a portion of the hammer circle adjacent the in-feed position, so that it can receive the particulates thrown off by the hammers. This member derives its name from the fact that impacting of the received particles against its surface causes further breaking of the particles. Also, this member has a surface or surfaces extending in a direction of rotation of the rotor and convergent with the hammer circle for crowding feed particulates against the rotor. The literature shows a wide variety of impact breaker members fabricated from castings and plates with regular or irregular surfaces and which may, for example, include depressions and jutting portions or may be generally arcuate, including truly arcuate surfaces or a series of flat surfaces arranged in an approximately arcuate fashion. Typically, the impact breaker member is fabricated in several individual sections for ease of installation or replacement.
Downstream of the impact breaker member, there is a cage which has a generally arcuate inner working face that confronts and is adjacent to the hammer circle. It includes a cage frame and plural grinding members supported in the frame. These may be distributed in the frame in one or more arrays for forming the working face. Typically these members are comminuting components which are to some extent elongated in the direction of and lie generally parallel to the rotor axis, meaning that they are more nearly parallel than perpendicular to said axis.
For example, such grinding members may be the comminuting components of single- or multi-piece grates, assemblies of bars, ridged plates or other forms of grinding members, and are mounted and distributed in or on the frame in a generally arcuate pattern at least partially surrounding the hammer circle. As applied to a grate assembly having both peripherally- and axially-extending grate elements, it is the axially-extending elements which are referred to herein as the grinding members, and it is of course these members which are referred to as lying generally parallel with the axis. More typically, the plural grinding members forming the working face of the cage are a series of bars lying substantially parallel to the rotor axis and distributed peripherally in the cage frame to form a working face of substantial area. Such bars are normally provided with spacers to keep the bars apart and to provide free and open communication between the hammer circle and the exterior edges of the bars. In a less typical arrangement, the grinding members may be ridges or other protrusions from or on the surface of an arcuate plate or casting, which may for example resemble a curved washboard. Regardless of the particular configuration of these grinding members, they are angularly spaced from one another about the axis when viewed in transverse cross section and have inner surfaces which confront and are adjacent to the hammer circle.
One popular and widely used reversible hammermill design known as the Pennsylvania.TM. reversible hammermill has been manufactured by the present inventors' assignee for many years prior to the present invention. In it, at least a portion of the grinding members are shearing members. These are typically distributed in the cage frame in a series, in which they are angularly and consecutively spaced about the rotor axis. Their purpose is to induce the major portion of the feed particulates traversing these shearing members to approach their inner surfaces obliquely, to abrade against their edges and, for the most part, to skip over such surfaces and continue downstream. This causes reduction of the particulates to occur primarily by shear forces (including abrasion) generated by glancing blows, as distinguished from impact reduction occasioned primarily by major changes in the velocity and/or direction of movement of the particulates, such as in the case of frontal collisions of particulates with an unmoveable obstacle. Thus, shearing type reduction usually results from a more oblique approach and collision than reduction with an impact breaker member. In the most recent form of the Pennsylvania.TM. hammermill extant prior to the present invention, the angular interval of the hammer circle subtended by said shearing members was less than 30 degrees.
In the Pennsylvania.TM. reversible hammermill, at least a portion of the grinding members are one or more groups of screening members which confront a portion of the hammer circle downstream of the shearing members. Typically, the angular widths of the screening members are about one inch or more and their angular spacing is about three-quarters of an inch or more. Typically, the ratio of angular spacing to angular width is about 0.5 or more, while the number of screening members per inch of working face (measured in the peripheral direction) is less than one. The screening members typically subtend an angular interval corresponding to at least about forty five degrees of the hammer circle. These screening members define a portion of the working face of the cage in which there is open communication between the hammer circle and the outer edges of the screening members. While further impact of particulates with the inner edges and faces of these screening members can and typically does result in some further reduction, including reduction by shearing forces, the distinctive function of these screening members is that they cause the major portion of the feed particulates which traverse them to exit the hammer circle via the spaces between the inner surfaces of the screening members.
For a number of practical reasons, the typical design approach for a Pennsylvania.TM. reversible hammermill has involved creation of a vertical axis of symmetry (on either side of a plane extending vertically through the axis of the rotor). This has certain advantages as explained by Hartshorn in U.S. Pat. No. 2,170,407, dated Aug. 22, 1939 and based on an application filed on Nov. 2, 1936. The typical design concept has also included dividing the machine into upper and lower portions delineated by an imaginary horizontal plane passing through the same axis or slightly above it. If a transverse cross-section of the machine is visualized as having a large clock face superimposed upon it with the center of the face coinciding with the rotor axis, this horizontal plane may be said to pass through the three o'clock and nine o'clock positions. In Pennsylvania.TM. reversible hammermills and other closely related equipment, it has been typical for the impact breaker member to be arranged along a portion of the hammer circle extending from about the one o'clock to three o'clock and nine o'clock to eleven o'clock positions. The grinding members, including the screening members and the relatively small expanse of shearing members heretofore employed have generally been distributed at and below the three and nine o'clock positions.
The foregoing arrangement, which has apparently been popular for about a half century (see the above-mentioned Hartshorn patent), has proven quite satisfactory and has been repeated over and over again in machine after machine. There seems to have been little if any dissatisfaction with this aspect of the design.